The present invention relates to an optical sensor for detecting a displacement and a displacement speed of a movable element in a musical instrument.
In natural musical instruments such as in a piano and a percussion instrument, a depression speed (or force) on a keyboard, or a force of a stick striking a head of a percussion instrument is the most important factor for producing a musical tone. When these natural musical instruments are formed into electronic instruments, the depression speed (force) is used as a parameter for producing a musical tone. For this purpose, demand has arisen for a sensor for accurately detecting a key depression force or a stick striking force. Various types of conventional sensors have been proposed.
In these musical instruments, however, such a sensor is arranged at a position where a musical tone is produced. In this sense, the mounting space of the sensor is often limited. For example, in an automatic playing piano or in an electronic musical instrument, sensors are respectively arranged near hammers operated in association with key depression operations or near keys arranged in a line. The mounting space of the sensor is limited to the width of each key along the horizontal direction and to a movable range of the key along the key depression direction, i.e., the vertical direction.
The sensor mounted in such a limited space must be compact, have high reliability and be easily maintained. In order to satisfy these needs, a combination of light-emitting and light-receiving elements can be utilized. In the combination, a shutter operated in association with key depression is arranged between a light-emitting element and a light-receiving element so as to cross the optical path between the light-emitting and light-receiving elements which are separated from each other.
A typical example is described in Japanese Patent Publication No. 56-33718 (issued on Jan. 27, 1977). A set of a light-emitting element 14 and a light-receiving element 15 is used, and a shutter 9 interlocked with depression of a key 1 is moved between the elements 14 and 15. More particularly, in FIGS. 1 to 3 of this Publication, when a player depresses the key 1, the key 1 is pivoted clockwise about a support member 2 as a fulcrum to move the shutter 9 upward to allow transmission of light L (FIG. 3). At the same time, a press member 3 is moved upward to pivot an arm 7 clockwise. Thus, a hammer 6 is pivoted together with the arm 7 to strike an upper portion 11a of a reflecting plate 11. When the hammer 6 strikes the upper portion 11a at high speed and returns to the initial position, the shutter 9 shields the optical path of the light L. The striking force of the hammer 6 changes in accordance with the strength of a depression force. Therefore, the same touch responsive characteristic as in a piano can be obtained. A time T (to be referred to as a shielding time) T for shielding the light L by the shutter 9 changes in accordance with the displacement speed of the hammer. More specifically, the higher the hammer speed is, the shorter the shielding time T is. Inversely, the lower the hammer speed is, the longer the shielding time T is. Therefore, the shielding time changes in accordance with the strength of key depression.
However, the combination consisting of the light-emitting and light-receiving elements comprises a large mechanism. Instead, a combination consisting of semiconductor elements can be made compact to some extent. However, a circuit board for mounting these elements is required. In addition, wirings on the circuit board and between the circuit board and other devices are required. As a result, the peripheral arrangement of the keys is complicated. In particular, when such a combination is replaced or wiring modifications are performed, the keys and the peripheral mechanism must be removed.